As a preliminary matter, the term ‘flat steel product’ is understood here as meaning steel sheets or steel strips produced by a rolling process and also sheet bars and the like cut off from said sheets or strips. Steel components of the type according to the invention are produced by a forming process from such flat steel products.
Unless otherwise expressly stated, whenever alloying contents are given here merely in “%”, this always means “% by weight”.
When reference is made here to “elongation at break A50”, “elongation at break A80” or “tensile strength Rm”, the mechanical characteristic values determined in accordance with DIN EN 6892-1 are meant.
Furthermore, U.S. Pat. No. 6,364,968 B1 discloses a method for producing a hot-rolled steel sheet which is intended to have a uniform distribution of its mechanical properties and particularly good hole-expanding characteristics in the case of a thickness of no more than 3.5 mm. The method thereby provides that a slab which comprises (in % by weight) 0.05-0.30% C, 0.03-1.0% Si, 1.5-3.5% Mn, up to 0.02% P, up to 0.005% S, up to 0.150% Al, up to 0.0200% N and alternatively or in combination 0.003-0.20% Nb or 0.005-0.20% Ti, is heated to up to 1200° C. and is then hot-rolled at a final hot-rolling temperature of at least 800° C., in particular 950-1050° C., into a hot strip. Then the hot strip obtained is cooled down at a cooling-down rate of 20-150° C./sec to a coiling temperature of 300-550° C., at which it is wound into a coil. The cooling down commences in this case within 2 seconds from the end of the hot rolling. The hot strip thus obtained is intended to have a fine bainitic microstructure with a bainite fraction of at least 90%, the average grain size of which does not exceed 3.0 μm, it being intended that the ratio of the length of the longest axis to the length of the shortest axis of the grains is no more than 1.5 and the length of the longest axis of the grains is no more than 10 μm. The remainder of the microstructure that is not taken up by the bainite is to consist of tempered martensite, which in its appearance and properties is very similar to the bainite. Hot strips produced in this way and of this form have tensile strengths of 850-1103 MPa with an elongation of 15-23%.
EP 2 546 382 A1 also discloses a method for producing a steel sheet with a tensile strength of at least 1470 MPa, in which the product of elongation and tensile strength is at least 29 000 MPa %. In addition to iron and unavoidable impurities, the steel of which the steel sheet consists in this case contains (in % by weight) 0.30-0.73% C, up to 3.0% Si, up to 3.0% Al, the sum of the Si and Al contents being at least 0.7%, 0.2-8.0% Cr, up to 10.0% Mn, the sum of the Cr and Mn contents being at least 1.0%, up to 0.1% P, up to 0.07% S and also up to 0.010% N. The steel sheet of such a composition is processed in such a way that the proportion by area of martensite in relation to the entire microstructure of the steel lies in the range of 15-90% and the amount of residual austenite contained in the microstructure is 10-50%. In this case, at least 50% of the martensite is intended to take the form of tempered martensite and the proportion by area of the tempered martensite is intended to be at least 10%. If they are present in the microstructure, at the same time the proportion by area of polygonal ferrites present in the microstructure should be at most 10%.
In order to achieve this, according to EP 2 546 382 A1 first a hot-rolled steel strip of the specified composition is produced by a preliminary steel material, such as a slab, being heated to 1000-1300° C. and, after that, rolled at a final hot-rolling temperature of 870-950° C. into a hot strip. The hot strip obtained is then wound into a coil at a coiling temperature of 350-720° C. After the coiling, a pickling is performed with subsequent cold rolling with degrees of deformation of 40-90%. The cold-rolled strip thus obtained is annealed for 15-1000 seconds at a temperature at which it has a purely austenitic microstructure, and is then cooled down at a cooling-down rate of at least 3° C./s to a temperature that lies in a temperature range beginning below the martensite start temperature and extending down to a temperature 150° C. lower, in order to produce tempered martensite in the microstructure of the steel sheet. After that, the cold-rolled steel strip is heated over a period of 15-1000 seconds to 340-500° C., in order to stabilize the residual austenite present. The cold-rolled steel sheets thus produced have achieved tensile strengths of more than 1600 MPa with an elongation of up to 27%.